The production of a small and deep hole or of a throughgoing opening of a diameter, say, 1 mm or less and a large depth-to-diameter ratio is extensively required in industry. For example, it is desirable to produce such holes or openings for drawing dies, fuel-injection nozzles in a diesel engine and fiber-spinning nozzles and also to form an initial hole in a workpiece to be machined by a traveling-wire or wire-cutting EDM operation.
It has generally been recognized that machining of these holes, openings or cavities can be achieved by electrical discharge machining with limited advantages because of various complicated measures and machining accessories required. A comparatively long machining time has been required for a comparatively small amount of stock removal and does not justify resort to the EDM technique for machining of small/deep hole formations, especially where the machining is to be carried out on a mass-production basis.